1. Field of the Invention
The present invention relates to a crystallization process for removing fluoride from waste water in an economic and efficient manner.
2. Description of the Prior Art
In various industries, including the production of semiconductors, chlorofluorocarbon (CFC) and glass, a large amount of fluoride-containing waste water with a high concentration of fluoride is produced. Therefore, many researchers have attempted to remove fluoride from the fluoride-containing waste water.
Jansen in U.S. Pat. No. 5,106,509 has disclosed a crystallization process for removing fluoride from waste water in a fluidized bed reactor. The process involves adding CaCl.sub.2 into waste water to react calcium ions and fluoride in the waste water to form calcium fluoride crystals. Compared with the coagulation/precipitation process that is utilized in factories to date, the advantages of the Jansen process are that the waste sludge is decreased and can be recycled. Since calcium fluoride has a very low solubility, such a crystallization process can effectively remove a great amount of fluoride. However, another result of the low solubility is that calcium fluoride will easily supersaturate in some locations, thus generating fine particles that can clog the pipes. For this reason, in practical use, waste water discharged from the factory with a high concentration of fluoride should be diluted to a concentration lower than 500 mgF.sup.- /l. In order to accommodate such a great amount of diluted waste water, the cost for the apparatus and for the land for accommodating the apparatus is increased.
Japanese Patent No. 8-11232 has disclosed a coagulation process for removing fluoride from waste water. The process involves mixing fluoride-containing waste water with aluminum and sodium ions to form cryolite coagulate as a means to remove fluoride. The residual fluoride contained in the treated waste water can be further removed. A water soluble aluminum salt is added into the treated waste water at pH 7 to form flocs aluminum hydroxide [Al(OH).sub.3 ], which can adsorb residual fluoride in the treated waste water to form a coprecipitate. The coprecipitate can be adjusted to be basic or acidic, thus forming an aluminum ion source for the coagulation process for removing fluoride from waste water according to the procedures as mentioned above.
In Japanese Patent No. 8-11232, the cryolite is in a form of coagulation. The coagulated sludge contains about 60.about.80% water, which is very difficult to dehydrate. In addition, the coagulated sludge is in a very large volume, and has many impurities; thus, it is not easily recycled.